U.S. Pat. No. 4,046,473 discloses a photocopy apparatus using a flexible endless photoreceptor belt which has a shuttle mechanism located between the imaging and processing stations for simultaneously storing and dispensing portions of the belt to enable the belt to be stopped at the imaging station during imaging and then rapidly removed therefrom after imaging while the velocity of the belt at the processing station remains constant. In this apparatus, the shuttle mechanism includes two displaceable rollers that are interconnected by two mutually parallel connecting arms which can be displaced in the plane of the rollers in a direction perpendicular to the rollers. When the speed of the photoreceptor belt in the imaging station differs from that in the processing station, the displaceable roller system will move in the direction perpendicular to the rollers in order to compensate for the speed difference in the belt.
A disadvantage of this apparatus is that the displacement of the movable roller system requires a complex construction comprising linear guides, counterweights and springs. A construction of this kind is by its nature slack and, accordingly, skewing can readily occur in the connected rollers if the belt is not ideally flat and linear. Moreover, fouling of the linear guides may also result in obstruction to the movement of the movable roller system.
Other similar devices are shown in Xerox Disclosure Journal, Vol. 8, No. 1, (January/February 1983) at pp. 51-52 and U.S. Pat. No. 3,495,903. The device shown in the Xerox Disclosure uses a dual vacuum tensioning chamber to buffer and equalize the different velocities in the belt portions in the xerographic machine. However, the vacuum chamber system of this device will show problems of excessive wear of the belt surface sliding over the chamber walls and of vibrations caused by eddies of air leaking into the chamber at the sides of the belt.
The device shown in U.S. Pat. No. 3,495,903 shows a way of changing web speed in one part of a device while maintaining a constant speed in the other part by moving the web rollers at the exposure station in a direction opposite to the motion of the web. In this device, the web itself is not driven at a different speed and thus control of the resulting speed differential is difficult because it involves controlling two different motions, i.e. the motion of the web and the displacement of the rollers. As a result, a very complicated mechanism that needs careful adjustment and that may wear easily is used to control the two different motions.
The present invention provides an image-forming apparatus without these disadvantages.